Data transmission using frequency shift keying (FSK) modulation of a radio frequency (RF) carrier wave is widely employed for transmitting digital data. A special case of spectrally efficient FSK is known as minimum shift keying (MSK). In MSK, two orthogonal signals represent the binary values 0 and 1. Typically a binary one is represented by a first frequency (f1) and a binary zero is represented by a second frequency which equals f2; the first and second frequencies have the same AC amplitude. Generators of MSK signals usually include an I-Q modulator having an input responsive to a binary data source and two mixers (that is, signal multipliers) responsive to orthogonal components of a carrier. The data rate of an MSK system is determined by the maximum frequency shift, i.e., frequency deviation or difference between f1 and f2. To preserve the orthogonal nature of MSK, the total frequency deviation equals the bit data rate divided by 2. For example, a typical very low frequency (VLF; between 3 kHz and 30 kHz) system in MSK mode with a frequency deviation of +/−50 Hz (i.e. 100 Hz total deviation) has a data rate equal to 100×2=200 bits per second. Any increase or decrease in data rate of an MSK system requires a corresponding change in frequency deviation.
MSK is often used in systems having transmit antennas with restricted useful bandwidth (typically 1 dB or less) because MSK is spectrally efficient. The wavelength of the RF carrier frequency frequently used in the VLF band is typically in the range of 10 to 30 kilometers. It is impractical to build a transmitting antenna large enough to be a significant fraction of these wavelengths. The typical VLF antennas, e.g., the antennas at the stations operated by the United States Navy in Maine and Hawaii for underwater radio communication, occupy about a thousand acres of land area and still are only a small fraction of a wavelength in height, despite having multiple transmitting towers that respectively have heights of 304 meters (997.5 feet) and 458.1 meters (1503 feet). The economics of land and construction costs put practical limits on the size of any high power VLF antenna; the tower in Maine radiates 1800 kilowatts of power at a frequency of 24.0 kHz, and can handle digital signals having up to a rate of 200 bits per second.
Based on the above, the useful bandwidth of a typical high power transmitter including a VLF antenna is much less than a typical transmitter having an antenna for higher frequency bands. The useful bandwidth of a typical transmitter including a VLF antenna is in the range of 25-100 Hz. The maximum data rate that can be transmitted by existing high power VLF transmitters is limited by the antenna system useful bandwidth of these transmitters.
One advantage of FSK and MSK is that the resulting RF signal has constant amplitude. Typical transmitter power levels for high power VLF transmitting stations are in the range of 100 kW to 2,000 kW. Therefore, high efficiency is a key requirement to minimize operational cost. Because the transmitted signal has a constant amplitude envelope it can be amplified by simple power amplifiers that operate in high efficiency modes, such as Class C or Class D. For this reason, all high power VLF transmitters utilize these types of high efficiency amplifiers and are incapable of handling any other type of modulation such as AM.
FIG. 1 is a block diagram of a typical prior art high power VLF transmitter employing MSK modulation. The transmitter of FIG. 1 is responsive to binary data source 910 having an output which supplies a bi-level, non-return to zero (NRZ) signal to MSK generator 912 which is responsive to VLF carrier source 916 and derives a frequency coded output, i.e., a variable frequency output dependent on the binary values of the output of source 910. In response to source 910 deriving binary one and zero values, generator 912 respectively derives first and second frequencies having the same AC amplitude at the carrier frequency and at 1.5 times the carrier frequency.
The MSK output of generator 912 is supplied to transmitter 914. Transmitter 914 includes a high power, high efficiency amplifier, such as a Class C vacuum tube amplifier including a tuned circuit having a resonant frequency equal to the VLF carrier frequency, or a Class D transistor amplifier including a low pass filter. Transmitter 914 also includes antenna impedance matching network 918, which is responsive to the output of the Class C or Class D amplifier, as appropriate.
If the data rate of source 910 is relatively low, no greater than 200 bits per second in the installations in Maine and Hawaii, network 918, in turn, supplies an MSK signal having an envelope with constant amplitude to high power VLF electromagnetic wave antenna system 920. Under such circumstances, antenna system 920, such as the previously described antenna systems in Maine and Hawaii, emits a VLF band wave with modulation having a substantially constant amplitude envelope with modulation having a wave shape that is a substantial replica of the wave shape derived by MSK generator 912.
The total frequency response of the cascaded sub-elements of the transmitter system of FIG. 1 can be found by taking the convolution of the impulse response of each of the sub-elements. In the block diagram of FIG. 1, antenna system 920 and matching network 918 cause the transmitter system of FIG. 1 to have an extremely narrow useful bandwidth. In the time domain, this narrow bandwidth causes errors in the transmitted waveform that increase rapidly with increasing data rate, particularly above 200 bits per second in the transmitters in Hawaii and Maine. The impulse responses of antenna system 920 and matching network 918 cause these errors in the time domain.
If the bit rate of source 910 is higher than a certain level, such as 200 bits per second, the components of transmitter 914, matching network 918, and particularly antenna 920 have frequency responses and group distortion (that is, an error in the relative time delay across the bandwidth of the antenna system 920 and the components between the antenna system and the output of generator 912) that change the shape of the frequency modulated wave which MSK generator 912 derives so that the shape of the modulation wave emitted by antenna system 920 is not a replica of the wave that generator 912 derives. Transmitter 914, matching network 918 and particularly antenna system 920 cannot accurately replicate the sidebands, especially the higher order sidebands, associated with accurate reproduction of the higher bit rate frequency modulated wave derived by MSK generator 912. (The reader will recall that a frequency modulated wave is theoretically represented by an infinite number of higher order terms having coefficients represented by Bessel functions.) Because the modulation wave emitted by antenna system 920 is not an accurate replica of the wave derived by generator 912, under these circumstances the signal at a receiver responsive to the wave emitted by antenna system 920 does not accurately replicate the output of binary data source 910.
Systems of the type illustrated in FIG. 1 have the disadvantages noted above relating to low data rate and are massive, highly expensive structures occupying enormous areas. In addition, considerable stresses are exerted on antenna system 920 in response to transients in the modulated wave the matching circuit supplies to the antenna system. For example, discharges sometimes occur across insulators of the antenna system, which insulators maintain components of the antenna system ungrounded.
It is, accordingly, an object of the present invention to provide a new and improved transmitting system, particularly adapted to operate in the VLF and/or low-frequency (LF; from 30 kHz to 300 kHz) ranges, wherein the transmitter has a relatively high rate of data transmission.
Another object of the invention is to provide a new and improved high power transmitting system, particularly adapted to operate in the VLF and/or LF band, wherein the system can employ an antenna system having reduced size and cost for a given data rate.
A further object of the invention is to provide a new and improved transmitting system, particularly adapted to operate in the VLF and/or LF range, wherein the transmitting system employs principles enabling it to be used in a large range of possible modulation types for various applications.
An additional object of the invention is to provide a new and improved high power transmitting system, which is particularly adapted to operate in the VLF and/or LF range, wherein the antenna system of the transmitter system has decreased voltage stresses, enabling the antenna system to radiate higher power.
An added object of the invention is to provide a new and improved high power transmitting system, which is particularly adapted to operate in the VLF and/or LF range, wherein changes in the antenna characteristics, due for example to weather and ground conductivity, are automatically compensated.